Most contact lenses on the market today are made of rigid gas permeable (RGP) plastics, hydrogels, or composite silicone-hydrogel materials. An essential requirement to contact lenses is oxygen permeability, so that the cornea of the eye, which lacks vascularization, has access to atmospheric oxygen and remains healthy when the lens is worn over a prolonged period of time. Materials have been developed that are optically transparent, durable, and have high oxygen permeability. A common requirement for a contact lens is that its oxygen permeability, Dk, must be higher than 10-20×10−9 cm/s×[mL O2/mL material×mmHg]. The highest oxygen permeability of the currently-used RGP plastics is ˜150×10−11 cm2/s×[mL O2/mL material×mmHg] (examples include MENICON Z® (Menicon Co. Ltd., Japan) and BOSTON XO2® (Bausch & Lomb, Inc., Rochester, N.Y.)), which limits the thickness of contact lenses that can be practically manufactured to ˜1 mm.
Scleral (>18 mm diameter) and mini-scleral (diameter 15-18 mm) lenses are configured to avoid contact with the cornea by vaulting the entire cornea and limbus. Such lenses have a number of advantages and benefits over contact lenses that are worn on the cornea (corneal contact lenses), hydrogel lenses and hybrid lenses, especially for a significant segment of patients with ametropia and ocular surface disease who struggle with limited wearing time or incomplete correction of their refractive error and presbyopia. Importantly, whereas corneal contact lenses are normally made of soft materials such as silicones and silicone/hydrogel composites, scleral contact lenses are usually made of RGP plastics. The relatively large size of scleral and semi-scleral lenses makes them a potentially more suitable platform for incorporation of multi-component optical assemblies. Such lens assemblies may include combinations of one or more filters with refractive lenses and/or diffractive lenses, and mirrors. The optimal thickness of such assemblies may exceed the ˜1 mm limit beyond which the oxygen permeability of presently-available RGP plastic becomes insufficient. Furthermore, construction of such assemblies may necessitate the use of oxygen-impermeable materials, which would block the transport of oxygen from the atmosphere to the cornea, making the lens unsuitable for extended wear.
Whereas composite lens assemblies are known in the art, the stated purposes of such lenses are to provide multiple optical modifications or adaptable optics rather than to improve delivery of oxygen to the cornea. For example, U.S. Pat. No. 6,851,805 of Blum et al. describes oxygen-permeable materials of which the inner and outer lens surfaces may be made as well as the creation of a rigid, non-permeable capsule configured to retain an electro-active lens. The issue of oxygen transport is not addressed in this patent. Both the capsule and the electro-active lens would interfere with oxygen transport, such that the oxygen exchange capability would be limited and not appreciably improved relative to a monolithic contact lens with the same external dimensions.
Other patents by Rosenthal (U.S. Pat. No. 8,087,777), Glorieux (U.S. Pat. No. 4,174,156) and Daphna (U.S. Pat. No. 8,096,655) describe layered lens constructions that include a cavity or chamber for retaining a fluid or gel. The oxygen permeability of aqueous solutions is about five orders of magnitude lower than that of air, or of a gas phase in general. As a result, inclusion of a fluid or gel within a cavity will necessarily impair the transport of oxygen through the cavity from atmosphere to the cornea.
Accordingly, the need remains for a contact lens construction that enables the advantages of versatile non-gas-permeable optical elements while providing sufficient transport of oxygen from atmosphere to the cornea, thus making the lens suitable for extended wear.